Fetal, Hormonal and Experiential Factors Influencing the Mating-induced Regulation of Infanticide in Male House Mice

When a male house mouse encounters a neonate he either attempts to kill it or he does not harm it. An unusual aspect of his response is that adaptive, time-dependent changes in behavior toward pups result from a unique stimulus-response system triggered specifically by ejaculation. In virgin male CF-1 mice, about 50% of all males are spontaneously infanticidal when they encounter a pup while the other 50% are typically "parental." The stimulus of ejaculation causes virtually all males to kill pups; however, by the time offspring are born three weeks after mating, infanticide is inhibited and almost all males now behave parentally toward pups. Our experiments examine fetal, experiential, and hormonal factors influencing the changes in infanticidal and parental behavior that occur in male mice as a result of mating. Males who developed in utero between two female fetuses, and were thus exposed to relatively low testosterone concentrations during fetal development, were significantly more likely to exhibit infanticide--both before and after mating--than were males who developed between two male fetuses. Concurrent exposure to testosterone appears required in order for naive males to exhibit infanticide and for spontaneously parental males to become infanticidal after ejaculation. In contrast, neither testosterone nor pituitary hormones appeared responsible for the timed inhibition of infanticide occurring by three weeks after mating, since castrated and hypophysectomized males showed a response pattern similar to intact males. The mating-induced inhibition phenomenon appears to be a neurally timed and mediated response that operates independently from pituitary hormone secretions or changes in gonadal hormones resulting from mating.